1. Field of the Invention
The present invention relates to transparent conductive films, particularly to cadmium-tin oxide films, and a method of producing same.
2. Description of the Prior Art
Metal oxide transparent conductive films which are now used for photoelectric converters such as solar cells and solid image pickup devices, or display devices such as liquid crystal display boards, can be grouped into three types: those of the type SnO.sub.2, those of the type In.sub.2 O.sub.3 and those of the type Cd.sub.2 SnO.sub.4. Among them, although the transparent conductive films of the types SnO.sub.2 and In.sub.2 O.sub.3 have long been studied, the transparent conductive films of the type Cd.sub.2 SnO.sub.4 have a recent history of study. It is only in recent years that the methods for their preparation and their characteristics have been reported as in Applied Physics Letters, Vol. 28, No. 10, p. 622, 1976; Physical Review, B, Vol. 6, No. 2, p. 453, 1972; and U.S. Pat. No. 5,811,953.
However, the transparent conductive films of Cd.sub.2 SnO.sub.4 have been formed by a high-frequency a-c sputtering method using a target made by molding under pressure and sintering a mixture of a Cd.sub.2 SnO.sub.4 powder or an SnO.sub.2 powder and a CdO powder, thus requiring cumbersome operation for preparing the target, also the handling of the apparatus for preparing the films to be sputtered and maintenance thereof, and eventually making it difficult to prepare the films having the desired compositions. It was therefore very difficult to prepare the transparent conductive films having the desired characteristics such as film thickness, specific resistivity, transmission factor of light, and temperature dependence of the resistance, while maintaining good reproduceability.
In applying transparent conductive films to the abovementioned various electronic devices, increasing demand has been placed on highly sophisticated characteristics such as low specific resistivity of the transparent conductive films, great transmission factor of light, low temperature dependence of the resistance, and uniform characteristics over wide areas.